Connacht captain Craig Clarke has been ruled out for an indefinite period after suffering his 10th concussion in 22 months.
The New Zealander joined Connacht last summer on a three-year deal after leading the Chiefs to back-to-back Super 15 titles, but will not play for the foreseeable future.
"Craig has had his tenth concussion in 22 months, so he is a big concern," said Connacht coach Pat Lam.
"At this stage he is going to be unavailable indefinitely, until there is a bit more testing and so forth.
"It is obviously a big blow for us but you can just not mess around with that sort of situation.
"The players had a week off after the Saracens game but he was suffering even last week. Obviously we favour investigations but more importantly is just time.
"I had a player before who has had that as well, Benson Stanley, and he was considering retiring and he just had to go through the timing and he was thinking about stepping down.
"But through timing and treatment and rest he got through and he is playing at Clermont now, so we are hopeful."